What is a MOT? Basically, it is a means of holding a cluster of neutral atoms suspended in a very small space, in a high vacuum. It is particularly useful for studying the decay products of radioactive atoms. Held in such a trap, these atoms are not in contact with any substrate with which they could interact, so we know the products observed arise only from the radioactive decay we are studying.

To make a MOT, we create a "damped harmonic oscillator", i.e. we not only provide a "spring" to confine the atoms, but we also slow the atoms' movements so that the spring can confine them to a small region. The slowing is produced by directing laser beams at the trapped atoms from all six directions. In general, when any atom absorbs radiation from one side, it is "pushed" in the opposite direction. The six lasers are tuned slightly to the red side of a resonance in the specific atoms being trapped.

The lasers also provide the confinement - except that we have not yet told the atoms where the centre of the trap is. To do that, we run current in opposite directions through the coils (see figure), so as to produce a magnetic field that reverses direction at the centre. Each atom has a magnetic moment in the direction of its spin (i.e. it behaves like a tiny bar magnet). The magnetic field tries to align the spin of the atoms, just like the earth's magnetic field orienting a compass needle, while the circularly polarized light (denoted sigma+ and sigma-) also tries to change the spin of the atoms. The net effect is that atoms above centre will preferentially absorb the downward-going light.

The same principles apply to atoms moving in other directions, and the result is that all these atoms are held near the centre of the trap.